In many business applications, color documents have become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of color output devices continue to look for ways to improve the total image quality of such devices. One of the elements that affects the perception of image quality is an ability to consistently produce the same quality image output on a printer from one day to another, from one week to the next, month after month. Users are accustomed to printers and copiers that produce high quality color and gray scale output. Users further expect to be able to reproduce a color image with consistent quality on any compatible output device, including another device within an organization, a device at home or a device used anywhere else in the world. Hence, there remains a commercial need for efficiently maintaining print color predictability, particularly as electronic marketing has placed more importance on the accurate representation of merchandise in illustrative print or display media.
Color rendering devices (e.g., a printer, copier, or other image output device) often have problems with maintaining accurate color outputs overtime due to many normally expected operational variations, e.g., printer drift, temperature and humidity variations, system aging, or the like. Accordingly, online, real-time calibration to maintain consistent and accurate color outputs is always a design and operational objective.
Inline spectrophotometric measuring systems for sensing reflectance vectors indicative of the colors produced by the color rendering device are well known, cf. U.S. Pat. No. 6,384,918 by Hubble III, et al.
Because real-time calibration is an important design objective, any embedded inline spectrophotometric measuring system must necessarily measure the colors on a printed substrate at a time before the substrate has cooled to an ambient temperature. Typically the measuring system is embedded at a location near the fuser so that the output is measured at a “just-fused” location within the output device and the substrate is at a temperature above where it will be when the print output has had an opportunity to cool to ambient temperature.
Recent data from operational studies of inline spectrophotometric systems suggest that color measurement differences occur between colors, when measured at the embedded location, with respect to similar measurements of the same prints made at ambient temperature. Such color measurement differences can be responsible for significant accuracy errors between the ultimately desired output color and the actual output color. The table below identifies empirically-determined error differences (“deltaE, or dE*”) in a range between a measurement at 60.0° C. and an ambient temperature of 22.0° C.
TABLE 1dE* from 22 deg C. (lab ambient)T (deg C.)K100B100C100M100P5255Paper White22.00.000.000.000.000.000.0025.00.150.170.210.200.250.0630.00.240.670.400.450.200.0535.00.190.200.820.240.300.3440.01.221.370.700.870.160.1045.01.911.460.971.810.140.1350.04.783.091.051.540.400.2055.03.732.301.932.560.510.1960.04.072.921.031.530.350.11
More particularly, it can be seen that the first or left-side column is the temperature of the printed substrate in degrees Centigrade ranging from 60.0° C. to 22.0° C. (60.0° C. is about the maximum temperature of the measured color at an embedded location near the fuser wherein the substrate has received an image and the image has just been fused thereon.) The entire table is relevant as a mapped reference for relating differences between the temperature of the measured color and ambient (22.0° C.). The table illustrates how the sensor reflectance vectors can vary with the change in temperature. The vertical columns represent one hundred percent saturated black (“K100”), blue (“B100”), cyan (“C100”), magenta (“M100”), a selected pantone color (“P5255”) and paper white. The table suggests that there are significant deltaE results for fused prints between the desired output color when it has cooled to ambient temperature, and what can be measured from the exact same substrated color at a higher temperature. For example, the deltaE for a fully saturated black, K100, has a 4.07 value difference from the exact same output and substrate at an ambient temperature of 22.0° C. If such a difference is not anticipated, and a compensation plan is not executed, color accuracy diminishes.
Accordingly, when an input signal is provided to the output device, which is supposed to generate a corresponding output color at an ambient temperature, the use of an inline spectrophotometric system measuring and relying upon only hotter colors, will not be able to verify that the color output is accurate and consistent with the intended color due to these naturally occurring thermochromaticity errors. The mistaken reliance on the measurement of a just-fused hot color to be the output cool color produces a mistaken, and inaccurate color printing system.
There is a need for a thermochromatic compensation system which can accommodate differences in color due to thermochromatic changes naturally occurring as a hot just-fused print substrate cools to an ambient temperature. Such a system would be useful to providing a more accurate and consistent color printing system for its compensation for thermochromatic measured errors, thereby increasing system robustness against thermal machine warm up, and the temperature drift due to normal machine aging or extended continuous job execution.
For the purpose of this invention, it is important to note that the errors between the measurements taken at a “just-fused” location within the output device, and when the print output had an opportunity to cool to ambient temperature are broadly grouped under “thermochromaticity error”, although the phrase “thermochromaticity” is referred specifically to chromatic shift occurring in color pigments with change in temperature. For example, there could be shift in lightness component (i.e., L*) of the color occurring when glossy images are cooled. We have grouped such kind of shifts occurring due to change in temperature as “thermochromaticity” errors.